Plasma-Wall Interactions with Nitrogen Seeding in all-Metal Fusion Devices: Formation of Nitrides and Ammonia

The use of N2 as a seeding gas for reduction of power loads onto the divertor targets of tokamak fusion reactors can have several implications due to the chemical reactivity of nitrogen. In this contribution recent experimental results on three topics are briefly presented. These are the formation of nitrides, the sputtering of beryllium by nitrogen and the production of ammonia by reaction with hydrogen. Laboratory experiments have shown that nitrides are formed both in beryllium (Be3N2) and tungsten (WN) upon exposure to energetic nitrogen ions. Considerable amounts of injected N can be stored in these nitrides. However, bulk material properties are not affected because the nitrogen uptake occurs only in a thin surface layer limited to the range of the energetic ions in the material. Erosion measurements by energetic nitrogen have been performed on beryllium and compared to modeling. The results exclude a strong chemical component in the investigated parameter range. Finally, the presence of ammonia in the exhaust gas of tokamak reactors has been observed. Quantification of the ammonia production rate requires disentangling the contributions from methane and water to the recorded mass spectra. The resulting production rate reached 5% of the injected N atoms in a series of three subsequent seeded discharges.
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